Refrigerator, and Method for Producing Very Low Temperature Cold

ABSTRACT

The invention relates to a method for producing very low temperature cold through a dilution cycle wherein a two-phase mixture of the two isotopes  3 He and  4 He is created in a mixing chamber from liquid  3 He and  4 He added separately, wherein said mixture of the  3 He from a so-called concentrated phase is extracted so as to pass  3 He into a so-called diluted phase, and due to which the cold energy generated by passing the  3 He into the diluted phase is recovered, the phase separation of the two-phase mixture being carried out by monitoring the flows of pure  3 He and  4 He, added separately into the mixing chamber, and by gravity-independent capillary forces, the dilution cycle operating in a closed loop, the method including a first step of recovering and separating the two isotopes  3 He and  4 He from the fraction of the mixture extracted through a discharge duct; and a second step of adding the two isotopes  3 He and  4 He, separated during the first step, back into the mixing chamber. The invention also relates to a refrigerator.

The present invention relates to a dilution refrigerator and to a method for producing very low-temperature cold.

The invention relates more specifically to a dilution refrigerator for obtaining very low temperatures comprising a mixing chamber, a first feed pipe having an upstream end connected to a source of helium isotope 3 (³He) and a downstream end connected to the mixing chamber, a second feed pipe having an upstream end connected to a source of helium isotope 4 (⁴He) and a downstream end connected to the mixing chamber, a discharge pipe for discharging the mixture of ³He-⁴He produced in the mixing chamber from the ³He and the ⁴He supplied respectively by the first and second pipes, the discharge pipe comprising an upstream end connected to the mixing chamber and a downstream end connected to a collecting volume for recovering the fraction of the discharged mixture, the downstream ends of the first and second pipes and the upstream end of the discharge pipe communicating fluidically at a common junction so as to form the mixing chamber, the phase separation between the mixtures of helium ³He-⁴He being controlled by the flows of ³He and ⁴He in the pipes and the capillary forces in the pipes independently of gravity.

Of the methods available for obtaining very low temperatures, one of the most advantageous involves diluting ordinary ⁴He with isotope ³He.

At below about 0.88K, the ³He-⁴He mixture may exhibit two phases, a ³He-rich phase known as the concentrated phase and a ⁴He-rich phase known as the dilute phase. When the temperature drops from 0.88K to 0K, the ³He concentration in the concentrated phase increases from 67% to around 100%, whereas the ³He concentration in the dilute phase decreases from 67% to around 6.6%.

A conventional dilution cooler conventionally comprises a mixing box or chamber filled with two phases of liquid helium: a phase which is a dilute phase and a phase which is a concentrated phase under the thermodynamic conditions described above. The principle for producing cold is essentially as follows: a ⁴He-³He mixture is created in the thermodynamically insulated mixing box in proportions such that the abovementioned two phases (a dilute phase and a concentrated phase) exist therein. By extracting ³He from the dilute phase, ³He from the concentrated phase will dissolve in the dilute phase in order to maintain the equilibrium concentration. This dilution process leads to the production of cold.

In order for such a device, or cryostat, to operate continuously, all that is required is for liquid ³He, possibly mixed with a little ⁴He, to be introduced into the mixing box in order to compensate for the withdrawals.

To allow these systems to operate, it is necessary to localize:

-   -   the concentrated phase (liquid containing predominantly ³He),     -   the dilution phase (liquid containing predominantly ⁴He), and         also     -   the concentrated vapor phase (predominantly ³He).

This localizing of the various phases is conventionally achieved under the effect of gravity (the phases separate because of their different densities). However, when the system is used under conditions of weightlessness, or in certain orientations, this form of localizing is not possible.

FR 2 626 658 describes a dilution cooler system that is independent of gravity or of the orientation.

In that system, use is made of three pipes also known as capillaries. The three capillaries are joined together at one end to form a junction (mixing chamber). Two capillaries are used respectively for injecting pure ³He and ⁴He in order to produce a saturated ³He-⁴He mixture (mixture of concentrated phase and of dilute phase). The helium mixture is extracted using the third capillary and is recovered or discharged into space which acts like a “free” pump. The phase separation between the liquid helium mixtures is controlled only by the flow of ³He and ⁴He and the capillary forces, and no longer controlled by gravity.

In order to circumvent the constraints associated with gravity, the refrigerator has no distiller. An ancillary distillation unit may be provided in order, during a dedicated operation, to separate the two constituent parts of the ³He-⁴He mixture that may have been recovered (for example by means of storage tanks).

The life of this system is, however, limited by the amounts of helium isotopes provided for feeding the mixing box. In addition, in the event of its being used in space craft, the extracted helium mixture is lost.

It is an object of the present invention to alleviate all or some of the abovementioned disadvantages of the prior art.

To this end, the refrigerator according to the invention, in other respects in accordance with the generic definition thereof given in the above preamble, is essentially characterized in that it comprises a still constituting the collecting volume in which the mixture is recovered, the still keeping the mixture of the ³He and ⁴He at liquid-vapor equilibrium, the still forming both the source of ³He and the source of ⁴He, the first feed pipe comprising a pumping member for selectively pumping ³He into the still in order to feed the mixing chamber with ³He continuously and in a first closed loop, the second feed pipe comprising a pumping member for selectively pumping ⁴He into the still in order to feed the mixing chamber with ⁴He continuously and in a distinct second closed loop.

Moreover, some embodiments of the invention may comprise one or more of the following features:

-   -   the still comprises a member for the containment of the liquid         phase with respect to the gaseous phase;     -   the pumping member for selectively pumping the ⁴He comprises a         superleak in fluidic communication with the liquid phase         contained by the containment member and a pumping member the         intake of which is connected to the liquid phase via the         superleak;     -   the first and second pipes and the discharge pipe are assembled         in order to exchange heat between the still and the mixing         chamber;     -   the pumping member of the first feed pipe is a pump for pumping         gaseous ³He, such as a mechanical pump and/or one or more pumps         known as absorption pumps;     -   the upstream end of the first feed pipe and/or the pumping         member of the first feed pipe opens into a zone of the still         that collects predominantly ³He from the ³He-⁴He mixture at         liquid-vapor equilibrium;     -   the pumping member of the second feed pipe is a pump for pumping         liquid ⁴He, such as a mechanical pump and/or a pump known as a         fountain pump (a thermo-mechanical pump) or a mechanical pump         for ⁴He;     -   the upstream end of the second feed pipe and/or the pumping         member of the second feed pipe opens into a zone of the still         that collects predominantly ⁴He from the ³He-⁴He mixture at         liquid-vapor equilibrium;     -   the pumping member of the second feed pipe or the upstream end         of the second feed pipe opens into a zone of the still via an         element for selective filtration of ⁴He.

The invention also relates to a method for producing cold at very low temperature, notably below 2K and more preferably below 1K using a dilution cycle in which a diphasic mixture of the two isotopes ³He and ⁴He is created in a mixing chamber from liquid ³He and liquid ⁴He which are introduced separately via respective feed pipes, in which method ³He from a phase known as the concentrated phase is extracted from said mixture, via a discharge pipe, in order to cause the ³He to enter a phase known as the dilute phase, and by virtue of which the cold energy generated by the ³He entering the dilute phase is recovered, the phases of the diphasic mixture being separated by controlling the flows of pure ³He and ⁴He introduced separately into the mixing chamber and the capillary forces in the pipes independently of gravity.

According to one advantageous feature, the dilution cycle operates in a closed loop, the method comprising:

-   -   a first step of recovering and separating the two isotopes ³He         and ⁴He from the fraction of mixture extracted by a discharge         pipe;     -   a second step of re-introducing the two isotopes ³He and ⁴He         separated during the first step into the mixing chamber.

According to other possible features:

-   -   the first step of recovering and separating the two isotopes is         carried out in a still configured to keep the ³He-⁴He mixture at         liquid-vapor equilibrium;     -   the second step of re-introducing the two ³He and ⁴He isotopes         into the mixing chamber is performed using respective pumping         members;     -   the mixture of the two isotopes ³He and ⁴He is kept in the         mixing chamber at a temperature comprised between 10 mK and 300         mK and for example between 50 mK and 300 mK;     -   the pumping pressure of the pumping member of the first pipe is         comprised between 0.1 and 50 mb, and is preferably equal to         approximately 5 mbar;     -   the method comprises, between the first, recovery and         separation, step and the second, re-introduction, step, a step         of respectively cooling one or each of the separated isotopes         between 1 and 2K and preferably between 1.4 and 1.5K;     -   the temperature in the still and the ³He concentration in the         still are kept such that the vapor pressure of ³He is far higher         than that of the ⁴He;     -   the delivery pressure of the pump of the first pipe is comprised         between 50 and 1500 mbar and is preferably of the order of 200         mb for liquefying the ³He at a temperature (pump outlet cooling)         of 1.4-1.5K;     -   the method uses a dilution refrigerator comprising a mixing         chamber, a first feed pipe having an upstream end connected to a         source of helium isotope 3 (³He) and a downstream end connected         to the mixing chamber, a second feed pipe having an upstream end         connected to a source of helium isotope 4 (⁴He) and a downstream         end connected to the mixing chamber, a discharge pipe for         discharging a fraction of the mixture of ³He-⁴He produced in the         mixing chamber from the ³He and the ⁴He supplied respectively by         the first and second pipes, the discharge pipe comprising an         upstream end connected to the mixing chamber and a downstream         end connected to a collecting volume for recovering the fraction         of the discharged mixture, the downstream ends of the first and         second pipes and the upstream end of the discharge pipe         communicating fluidically at a common junction so as to form the         mixing chamber, the phase separation between the mixtures of         helium being controlled by the flows of ³He and ⁴He and the         capillary forces in the pipes rather than by gravity, the         refrigerator further comprising a still constituting the         collecting volume in which the mixture is recovered, the still         keeping the mixture of the ³He and ⁴He at liquid-vapor         equilibrium, the still forming both the source of ³He and the         source of ⁴He, the first feed pipe comprising a pumping member         for selectively pumping ³He into the still feeding the mixing         chamber with ³He continuously and in a first closed loop, the         second feed pipe comprising a pumping member for selectively         pumping ⁴He into the still feeding the mixing chamber with ⁴He         continuously and in a second closed loop;     -   the pressure in the pump of the second pipe (the fountain         pressure pump) may be several hundred mbar in order to have a         turbulent flow in the monophasic capillaries and in the         capillary between said fountain pressure pump and the still;     -   when the pump 7 of the second pipe 4 is a mechanical pump, the         pumping pressure may be negative.

The invention may relate also to any alternative device or method comprising any combination of the features listed hereinabove or hereinbelow.

Other specifics and advantages will become apparent from reading the following description, which is given with reference to the FIGURE which schematically and partially depicts the structure and operation of a refrigerator according to one possible embodiment of the invention.

The dilution cooler 1 comprises a mixing chamber 2 formed at the open ends (common junction) of a first ³He feed pipe 3, of a second ⁴He feed pipe 4, and of a discharge pipe 8 for discharging the ³He-⁴He mixture.

The discharge pipe 8 comprises, from upstream to downstream, two portions: a first portion (the zone referenced 13) through which the two phases (concentrated and dilute) pass and a second portion (the zone referenced 12) through which the monophasic ³He-⁴He mixture passes once the concentrated phase has been fully diluted in the dilute phase. This discharge pipe 8 therefore serves to discharge the diphasic ³He-⁴He mixture produced in the mixing chamber 2 from the ³He and the ⁴He supplied respectively by the first 3 and second 4 pipes.

The phase separation between the mixtures of helium ³He-⁴He is controlled by the flows of ³He and ⁴He through the pipes 3, 4, 8 and the capillary forces in the pipes 3, 4, 8. What that means to say is that the phase separation is dependent neither on gravity nor on the orientation (according to the same general principle as was employed in document FR 2 626 658).

The refrigerator 1 comprises a still evaporator 5 containing the dilute liquid ³He-⁴He mixture in equilibrium with the ³He-rich vapor phase.

The still 5 is, for example, a copper and/or stainless steel subassembly with the required fluid inlets and outlets and which constitutes a sealed volume of a few cubic centimeters for example. The volume of the still 5 is sized so that the liquid-gas interface is established inside said volume, according to the quantities of He passing through the system.

The upstream end of the first pipe 3 is connected to the still 5 via a pump 6 and the downstream end of the first pipe 3 is connected to the mixing chamber 2.

The second feed pipe 4 has an upstream end connected to the still 5 via a pump 7 and a downstream end connected to the mixing chamber 2.

The discharge pipe 8 is connected to the mixing chamber 2 by its upstream end and is connected to the still 5 by its downstream end.

The apparatus 1 therefore forms two closed loops between the still 5 and the mixing chamber 2. The apparatus 1 is filled with a saturated ³He and ⁴He mixture so that there is a vapor-liquid interface in the still 5 and a concentrated—dilute interface in the mixing chamber 2.

The first feed pipe 3 feeds the mixing chamber 2 with ³He from the still 5 via a pumping member 6 such as a pump.

The second feed pipe 4 feeds the mixing chamber 2 with ⁴He from the still 5 via a pumping member 7 such as a pump.

The pump 6 of the first pipe 3 pumps predominantly ³He (gaseous) because the temperature in the still 5 and the ³He concentration in the still 5 are kept such that the vapor pressure of the ³He is far higher than that of the ⁴He.

This pump 6 may be a mechanical pump or any other suitable equivalent pumping system placed at ambient temperature or at cryogenic temperature (for example an adsorption pump). After being pumped from the still 5, the ³He is cooled before it is introduced into the mixing chamber 2. For example, a cooler 10 liquefies the ³He pumped in gaseous form from the still 5. This cooler 10 may, for example, be made up of a Joule-Thomson expansion system operating on He (³He or ⁴He) or any cooler able to supply a temperature ideally of the order of 1.4 to 1.5K. After this first cooling 10, the ³He is cooled by the still 5 (exchange of heat with the first pipe 3). Next, the ³He may be cooled by exchange of heat between the first pipe 3 and the discharge pipe 8 (this discharge pipe 8 potentially also being in a heat exchange relationship with the second pipe 4). There is thus a zone 12 in which the injected liquid ³He exchanges heat with the monophasic ³He-⁴He mixture and then a zone 13 for exchange of heat between the injected liquid ³He and the diphasic ³He-⁴He mixture.

The helium isotope 3 (³He) injected in liquid form into the mixing box 2 typically has a temperature comprised between 10 mK and 300 mK.

The pump 7 of the second pipe 4 pumps exclusively liquid ⁴He. The liquid ⁴He pump 7 may be connected to the still 5 by means for example of a system 9 known as a superleak which acts like a semipermeable membrane allowing only the superfluid ⁴He to be pumped. This superleak 9 has one end 19 immersed in the liquid phase of the still 5 and one end 29 not immersed and preferably thermally insulated from the still 5. Furthermore, a boundary device 14 may be used to allow containment of the liquid ³He-⁴He mixture in contact with the immersed end 19 of the superleak. This boundary device 14 may operate using capillarity, for example it may consist of a porous medium with a pore size distribution suited to the intended application. Other systems, for example employing an electric field, may be imagined for achieving this containment between the liquid phase and the gaseous phase.

The pump 7 may be a fountain pump (a thermomechanical pump) or a mechanical ⁴He pump or any other appropriate equivalent member positioned as appropriate downstream of the superleak 9 at ambient or cryogenic temperature (for example an adsorption pump or a cold turbine).

The superfluid ⁴He pumped from the still 5 may be cooled by an external cold source 11 which performs the same function as the cooler 10 of the first pipe 3 (cooling to a temperature ideally of the order of 1.4 to 1.5K). The two coolers 10 and 11 may incidentally form just one single same cooling element. It will be noted that it is thermodynamically conceivable to dispense with a cold source (cooler 10 and/or cooler 11, either or both of which may be optional), particularly if the dissipation of heat by the pump 7 of the second pipe 4 is sufficiently low.

Following this first cooling 11, the ⁴He may be cooled by the still 5 (exchange of heat with the second pipe 4). Next, the ⁴He is cooled by exchange of heat between this second pipe 4 and the discharge pipe 8 (this discharge pipe 8 possibly also being in a heat exchange relationship with the first pipe 3). There is thus a zone 12 in which heat is exchanged between the injected liquid ³He and the monophasic ³He-⁴He mixture then a zone 13 in which heat is exchanged between the injected liquid ³He and the diphasic ³He-⁴He mixture.

The helium isotope 4 (⁴He) injected in liquid form into the mixing chamber 2 typically has a temperature comprised between 10 mK and 300 mK.

The operating temperatures in the mixing chamber 2 are generally comprised in the range from 10 mK to 300 mK.

For the fountain pressure pump 7 of the second pipe 4 to be able to operate effectively, the ³He concentration in the liquid phase of the still 5 is preferably of the order of 10%, the temperature in the still 5 is preferably around 1.05K. As a result, the vapor pressure in the still 5 is of the order of 5 mb and the ³He concentration in the vapor is close to 95%.

The pumping pressure of the pump 6 of the first pipe 3 is therefore typically of the order of 5 mb and the delivery pressure is, for example, of the order of 200 mb or higher, and this allows the ³He to be liquefied at the temperature of 1.4-1.5K.

The temperature of the fluids after pumping and first cooling 10, 11 is, for example, comprised between 1 and 2K.

The dilution refrigerator 1 according to the invention therefore uses a containment system 14 that allows the liquid and vapor phases in the still 5 to be localized.

The refrigerator 1 according to the invention thus makes it possible continuously to maintain two distinct flows of helium isotopes (⁴He and ³He) in two closed loops, without the need to add helium from outside.

The refrigerator 1 or cryostat obtained allows a stable temperature of the order for example of 0.05K to be produced and maintained in the mixing chamber 2 with unlimited autonomy.

The system described hereinabove may be mounted on a support that can be oriented in all directions with a view notably to applications in a weightlessness environment or those subjected to a gravitational field. 

1-16. (canceled)
 17. A dilution refrigerator for obtaining very low temperatures comprising: a mixing chamber; a first feed pipe having an upstream end connected to a source of helium isotope 3 (³He) and a downstream end connected to the mixing chamber, a second feed pipe having an upstream end connected to a source of helium isotope 4 (⁴He) and a downstream end connected to the mixing chamber; a discharge pipe for discharging the mixture of ³He-⁴He produced in the mixing chamber from the ³He and the ⁴He supplied respectively by the first and second pipes, the discharge pipe comprising an upstream end connected to the mixing chamber and a downstream end connected to a collecting volume for recovering the fraction of the discharged mixture, the downstream ends of the first and second pipes and the upstream end of the discharge pipe communicating fluidically at a common junction so as to form the mixing chamber, the phase separation between the mixtures of helium ³He-⁴He being controlled by the flows of ³He and ⁴He in the pipes and the capillary forces in the pipes independently of gravity; and a still constituting the collecting volume in which the mixture is recovered, the still keeping the mixture of the ³He and ⁴He at liquid-vapor equilibrium, the still forming both the source of ³He and the source of ⁴He, the first feed pipe comprising a pumping member adapted to selectively pump ³He into the still in order to feed the mixing chamber with ³He continuously and in a first closed loop, the second feed pipe comprising a pumping member adapted to selectively pump ⁴He into the still in order to feed the mixing chamber with ⁴He continuously and in a distinct second closed loop.
 18. The refrigerator of claim 17, wherein the still comprises a member for the containment of the liquid phase with respect to the gaseous phase.
 19. The refrigerator of claim 18, wherein the pumping member adapted to selectively pump the ⁴He comprises a superleak in fluidic communication with the liquid phase contained by the containment member and a pump the intake of which is connected to the liquid phase via the superleak.
 20. The refrigerator of claim 17, wherein the first and second pipes and the discharge pipe are assembled in order to exchange heat between the still and the mixing box.
 21. The refrigerator of claim 17, wherein the pumping member of the first feed pipe is a pump for pumping gaseous ³He selected from the group consisting of a mechanical pump, one or more adsorption pumps, and combinations thereof.
 22. The refrigerator of claim 17, wherein the upstream end of the first feed pipe and/or the pumping member of the first feed pipe opens into a zone of the still that collects predominantly ³He from the ³He-⁴He mixture at liquid-vapor equilibrium.
 23. The refrigerator of claim 17, wherein the pumping member of the second feed pipe is a pump for pumping liquid ⁴He selected from the group consisting of a mechanical pump, a thermo-mechanical pump, a superfluid helium mechanical pump, and combinations thereof.
 24. The refrigerator of claim 17, wherein the upstream end of the second feed pipe and/or the pumping member of the second feed pipe opens into a zone of the still that collects predominantly ⁴He from the ³He-⁴He mixture at liquid-vapor equilibrium.
 25. The refrigerator of claim 23, wherein the pumping member of the second feed pipe or the upstream end of the second feed pipe opens into a zone of the still via an element for selective filtration of ⁴He.
 26. A method for producing cold at very low temperature, using a dilution cycle in which a diphasic mixture of the two isotopes ³He and ⁴He is created in a mixing chamber from liquid ³He and liquid ⁴He which are introduced separately via respective feed pipes, in which method ³He from a phase known as the concentrated phase is extracted from said mixture, via a discharge pipe, in order to cause the ³He to enter a phase known as the dilute phase, and by virtue of which the cold energy generated by the ³He entering the dilute phase is recovered; the phases of the diphasic mixture being separated by controlling the flows of pure ³He and ⁴He introduced separately into the mixing chamber and the capillary forces in the pipes independently of gravity, characterized in that the dilution cycle operates in a closed loop, the method comprising the steps of: recovering and separating the two isotopes ³He and ⁴He from the fraction of mixture extracted by a discharge pipe; and re-introducing the two isotopes ³He and ⁴He separated during the first step into the mixing chamber.
 27. The method of claim 26, wherein said step of recovering and separating the two isotopes is carried out in a still configured to keep the ³He-⁴He mixture at liquid-vapor equilibrium.
 28. The method of claim 26, wherein said step of re-introducing the two ³He and ⁴He isotopes into the mixing chamber is performed using respective pumping members.
 29. The method of claim 26, wherein the mixture of the two isotopes ³He and ⁴He is kept in the mixing chamber at a temperature between 10 mK and 300 mK.
 30. The method of claim 26, wherein a pumping pressure of the pumping member of the first pipe is between 0.1 and 50 mb.
 31. The method of claim 26, further comprising, between said step of recovery and separation and said step of re-introduction, a step of respectively cooling one or each of the separated isotopes between 1 and 2K.
 32. The method of claim 26, wherein said method is performed to produce cold energy at a temperature below 2K.
 33. The method of claim 26, wherein said method is performed to produce cold energy at a temperature below 1K.
 34. The method of claim 30, wherein the pumping pressure of the pumping member of the first pipe is approximately equal to 5 mbar.
 35. The method of claim 26, further comprising, between said step of recovery and separation and said step of re-introduction, a step of respectively cooling one or each of the separated isotopes between 1.4 and 1.5K.
 36. The method of claim 26, wherein said method utilizes a dilution refrigerator comprising a mixing chamber, a first feed pipe having an upstream end connected to a source of helium isotope 3 (³He) and a downstream end connected to the mixing chamber, a second feed pipe having an upstream end connected to a source of helium isotope 4 (⁴He) and a downstream end connected to the mixing chamber, a discharge pipe for discharging a fraction of the mixture of ³He-⁴He produced in the mixing chamber from the ³He and the ⁴He supplied respectively by the first and second pipes, the discharge pipe comprising an upstream end connected to the mixing chamber and a downstream end connected to a collecting volume for recovering the fraction of the discharged mixture, the downstream ends of the first and second pipes and the upstream end of the discharge pipe communicating fluidically at a common junction so as to form the mixing chamber, the phase separation between the mixtures of helium being controlled by the flows of ³He and ⁴He and the capillary forces in the pipes rather than by gravity, the refrigerator further comprising a still constituting the collecting volume in which the mixture is recovered, the still keeping the mixture of the ³He and ⁴He at liquid-vapor equilibrium, the still forming both the source of ³He and the source of ⁴He, the first feed pipe comprising a pumping member for selectively pumping ³He into the still feeding the mixing chamber with ³He continuously and in a first closed loop, the second feed pipe comprising a pumping member for selectively pumping ⁴He into the still feeding the mixing chamber with ⁴He continuously and in a second closed loop. 